If you're anywhere near Dayton Ohio, then a day-long stop at the Wright-Patterson Air Force Museum is a must. It is currently the largest and best displayed military aviation museum in the world. If you like military aircraft, you will never regret a visit here.
Wash D.C. of course has many you may wish to consider. If you find yourself on the road in the Eastern part of the U.S., you may want to look up civil was sites, *****sburg; Antietam; Shiloh; etc. If in the West you may wish to stop look into Western expansion - forts; railroads; cattle drives; wagon trains; etc., or look at some of the state museums where you are traveling, usually in the capitals.
I visited the Midway last summer. I planned 2 hours for it but ended up spending the whole day there. Worth every single second of it. And it's in the middle of a big city, so you don't need to drive all the way to some random place like Quantico just to see a really nice museum.